The peculiarities and advantages of the present invention will be more evident after a short description of the state-of-the-art.
With reference to FIG. 1, the current models of washers and washer/dryer combos are provided with a plastic tub with composite structure, being formed of two separate parts (1, 2) designed to be securely coupled by means of fixing means of known type.
The first part consists in a large concave cylindrical body (1) closed at one end by a bottom wall (10) with a central hole, connected on the back with a cylindrical nozzle (11) designed to exactly receive a metal bearing support with basically cylindrical shape.
The second part consists in a flange (2) provided with a large central opening (20a) designed to be mounted at the front open end of the concave cylindrical body (1).
Such a tub usually contains a cylindrical drum (3) designed to be loaded with dirty laundry through the opening (20a) of the front closing flange (2) of the tub.
The drum (3) is closed on the back by a bottom wall (30) with an axially protruding shaft (31) in central position, designed to be driven into rotation by the electrical motor that is normally mounted in the household appliance outside the tub.
In particular, the shaft (31) is designed to be inserted and supported, with the interposition of a pair of ball bearings with different diameter, inside the bearing support inserted in the nozzle (11) on the bottom wall (10) of the concave cylindrical body (1) of the tub.
The shaft (31) of the drum (3) is driven into rotation by a motor fixed in external position on the tub.
Moreover, such a traditional tub is fixed inside the cabinet of the washer by means of suitable elastic means, which are also designed to act as shock absorbers for the significant mechanical stress transmitted to the tub during the rotations of the drum, especially during spinning.
The presence of the elastic elements avoids the damages caused by the high stress to the structure of the tub.
The aforementioned tub-assembly is traditionally identified as “Oscillating Assembly”.
The tendency of the laundry loaded inside the drum (3) to be arranged in a totally random way during the rotation of the drum (3) originates an unbalanced rotational mass associated with a centrifugal force that can reach very high values.
To counterbalance the centrifugal force and avoid the contact with the cabinet, the oscillating assembly must be ballasted up to a total weight normally ranging between 32 and 60 kg.
To reach such a weight counterweights (normally two, sometimes one or three) usually made of concrete must be anchored to the tub.
With reference to FIG. 2, to favour the anchoring and centring of the said counterweights (21), the cylindrical body (1), as well as the corresponding front flange (2), is externally provided with one or more cylindrical columns (22), each of them being internally provided with an axial rectilinear conduit (22a) designed to receive a corresponding self-tapping screw (24).
As shown in FIG. 2, each column (22) protrudes from a corresponding enlarged base with cylindrical shape (23).
Each counterweight (21) is provided with a through hole (21a), with basically truncated-conical shape, designed to receive one of the said columns (22) when the counterweight is engaged against the upper side of the corresponding enlarged base (23).
In this way the upper end of each column (22) can be accessed from the upper opening of the hole (21a) of the counterweight (21) to allow for engagement with the self-tapping screw (24).
The self-tapping screw (24) is generally provided with a large washer (26) designed to be energetically engaged around the upper opening of the hole (21a) of the counterweight (21).
The careful examination of a similar prior technique has shown critical aspects that can be remedied by the present invention.
A first severe limitation of the said traditional columns (22) relates to the operation principle.
Reference is made to the fact that the mutual coupling between the column (22) and the hole (21a) of the counterweight (21) is obtained because of the compression and permanent deformation of the plastic material of the column (22).
More precisely, the said deformation is produced in the upper end of the enlarged base (23) of each column (22) because of the energetic interference produced by the lower opening of the hole (21a) of the counterweight (21) following to the progressive tightening of the screw for mutual fixing (24).
In fact, due to the tightening of the screw, the plastic material of the base (23) of the column (22)—being generally reinforced or loaded polypropylene material—tends to be moulded and forcedly adhered against the surface of the counterweight (21).
In this condition the anchoring of the counterweight is not completely satisfactory, since the counterweight may be moved from its operational position without control.
Such a case could occur, for example, in case of anomaly of the electronic system that controls the rotation of the said drum with the laundry.
In case of such an anomaly, the inertial force generated on the oscillating assembly of the washer may be so high that it cannot be efficaciously opposed by the front interference established between the plastic material of the base (23) and the concrete material of the counterweight (21).
Moreover, the traditional columns (22) have a very complex geometry that requires the use of a large quantity of plastic material as well as long cooling time at the end of the moulding operations.
In view of the above, the parts of the tub provided with the said columns (reference is made to the cylindrical body and corresponding closing flange) cannot be extracted from the corresponding moulds, although perfectly cooled, until the longer cooling process of the columns has been completed.
Because of the said cooling difficulties, undesired unexpected internal cavities may be formed on the structure of the columns, thus impairing the resistance and correct fixing with the counterweights.
In the worst cases the presence of the said internal cavities totally impairs the capability of a column to guarantee the anchoring of a corresponding counterweight because the self-tapping screw engaged inside it cannot reach the expected torque and loses its thread.